out of the trenches

'but the moon has no seas' was also my first thought... but equally it's unfair measuring the height of Everest from the bottom of Mariana Trench.

The trench is a localised feature. Ideally the depth of trenches and height (or rather 'tallness') or mountains should be measured from the mean surface elevation (which itself I suppose is measured from the gravitational centre of the planetoid?)

in other words the depth of a trench shouldn't count towards the tallness of a mountain

I wonder if NASA have already done that in this case, since they talk about a 'lunar surface' which is a rather vague term